Effects of a barrage on flushing and water quality in the Fitzroy River estuary, Queensland

Abstract

Tidal exchange was found to flush the Fitzroy River estuary at a comparatively slow rate, leading to residence times of substances in the estuary of approximately 8 months both before and after barrage construction. The principal mechanism of flushing is displacement by freshwater inflow which gives residence times of 0.2-3.5 months. Freshwater inflow has been decreased by barrage construction and residence times consequently increased.

Salinity and dissolved oxygen, total phosphorus, filterable reactive phosphorus, oxidized nitrogen and chlorophyll a concentrations were measured at stations located 20 to 110 km upstream from the mouth of the Fitzroy River. This included that section containing the barrage which was constructed 5 km upstream from Rockhampton. Under conditions of high river discharge (22.6 × 10⁸ m³) little variation in water quality was found, but under low discharge conditions (0.035 × 10⁸ m³) substantial reductions in dissolved oxygen concentration and increases in nutrient and chlorophyll a concentrations were noted in the zone below the barrage. These results are indicative of the impact of barrage construction on water quality, but more detailed chemical and biological investigations would be needed to unequivocally evaluate water quality status.